1. Field of the Invention
This invention relates to a method of producing fine-line circuitry on the surface of a printed circuit board having plated through holes. Specifically, full additive process of circuitization is employed to produce a printed circuit board having fine-line circuitry and increased density. This invention also relates to the resulting printed circuit board and its use in surface laminar circuit technology to form composite structures.
2. Description of Related Art
A conventional process for producing printed circuit boards used in composite structures is as follows: A pattern of holes is drilled through a dielectric substrate or panel. The substrate panel surface and the surfaces created by the holes of the drilled substrate are seeded with a palladium/tin colloidal suspension to catalyze the surfaces for electroless copper plating. Next, the dielectric substrate is copper plated, via electroless or electrolytic deposition, onto the panel surfaces and onto the surfaces created by the through holes at a thickness of approximately 1 mil to about 1.5 mils to form a subcomposite. A minimum thickness copper is required to prevent stress cracking in the through holes. This minimum thickness of copper plating of about 1 mil is often thicker than the desired thickness of the circuitry to be formed on the panel surface, and thus, the subcomposite may be partially etched to reduce the copper thickness on the panel surfaces to about 0.5 mil. Next, the plated through holes are then filled with a compound that can be either conductive or nonconductive. Typically, a layer of negative-acting photoresist is applied to the subcomposite and the photoresist is exposed and developed to define a desired pattern of surface circuitization. The revealed portion of the of copper is then etched away, and finally, the photoresist is stripped to reveal the desired pattern of circuitry.
The subtractive etch method of circuitization, described above, yields a subcomposite having limited wiring density. When conductive metal is etched to form spaces between lines of circuitry, the conductive metal which is covered by photoresist during formation of circuitry, is susceptible to erosion by the etchant in areas beneath the photoresist. To avoid this problem, the thickness of the circuit lines divided by the width of the spaces between them, i.e. the aspect ratio, must be sufficiently low to produce a functional product. The width of spaces limits wiring density. For example, a typical circuit board that has circuit lines about 1 mil thick, requires line and space widths of at least about 3 mils. Also, sharp edge definition, such as the formation of squared lines, is difficult to achieve using the subtractive etch method.
Accordingly, the need exists for a printed circuit board and method of making printed circuit boards, having higher aspect ratio and fine-line circuitry definition on subcomposites with filled plated through holes.